I don't remember mermaids becoming huge serpents when in water, she looks more like a leviathan or something? Well, atleast that would make her wanting to eat her(in the literal way) make more sense than a mermaid.
@Chizan
The beach woman called her a mermaid, and then in the later panels, a huge serpent being rips the beach woman to shreds, and then Shiori comes back with blood all over. I'd believe they know what they're talking about so they're not wrong about what kind of monsters they are, I just thought mermaid seems off for what Shiori seems to be. Well, she might be called a mermaid just for the fact she can disguise herself as a normal woman, I suppose.
I'm glad I wasn't the only one to think about Touko and Yuu while reading this. Anyway, this looks interesting. I love the art and the creepy feeling. That first colored page is beautiful. Count me in (ง'̀-'́)ง